1. Field of the Invention
This invention generally relates to a semiconductor device, and more particularly to a silicon controlled rectifier (SCR) for electrostatic discharge (ESD) protection.
2. Description of Related Art
Semiconductor integrated circuits (ICs) or devices are normally designed with at least an additional protection structure or circuit to provide a discharge path for the additional current caused during high-amplitude electrostatic discharge (ESD) pulses, which can effectively prevent damages to the device or the IC. ESD protection circuits or devices are usually located between the input/output (I/O) pads (or pins) and ground (or power rail). Moreover, ESD protection devices can be incorporated into the MOS circuits on the chip.
The silicon controlled rectifier (SCR) devices have been proposed to serve as the ESD protection device due to its relatively high ESD level and small layout area. The SCR device is made up of four semiconductor layers arranged as P-N-P-N and can be considered as one PNP transistor and one NPN transistor working together. As an ESD stress having a voltage higher than the triggered point (or the turn-on voltage) is applied to the anode, the SCR device becomes conductive and permits the majority of the ESD current to bypass, thus enhancing the ESD tolerance of the circuits.
In principle, when the semiconductor device is in normal operation, the ESD protection structure should not be activated unexpectedly. For the semiconductor device having a dual polarity input/output (I/O) pad, the detection of the static electricity can be classified into a positive polarity situation and a negative polarity situation. However, as the pn junction of the existent ESD protection structure is only capable of forming a forward-bias diode, the semiconductor device will not be satisfactorily protected under the negative polarity situation.
Therefore, the ESD protection device or circuits should be designed to protect the semiconductor device against any electro-static current regardless of whether the electro-static current is a positive electro-static current or a negative electro-static current.